It is well-known that bisphenol A is an important compound as a starting raw material for epoxy resin or an engineering plastic such as polycarbonate resin and polyarylene resin, and accordingly it tends to increasingly expand its demand year by year.
Bisphenol A is produced by the condensation reaction of excess phenol with acetone in the presence of an acidic catalyst and as the case may be, a sulfur compound etc. as a cocatalyst.
As a process for singling out bisphenol A from a reaction mixture, there are known a process comprising directly separating the same in the form of coarse crystal from the reaction mixture and a process comprising removing acetone, water and the like from the reaction mixture to leave a liquid mixture, thereafter concentrating cooling the resultant liquid mixture to precipitate an adduct of bisphenol A with phenol and separate the same into each other {(refer to Japanese Patent Application Laid-Open No. 91240/1976 (Show 51), Japanese Patent Application Laid-Open No. 77637/1982 (Show 57) and the like}.
The former process comprising directly separating bisphenol A in the form of coarse crystal from the reaction mixture suffers from such disadvantages as necessitating several times of cleaning owing to the microcrystalline property thereof, whereby product loss is increased.
Such being the case, there is prevalently carried out at the present time, the latter process comprising precipitating an adduct of bisphenol A with phenol and separating the same. In this case, the adduct of bisphenol A with phenol is crystallized, and the resultant crystal is separated form a mother liquor by a publicly well known solid-liquid separation method using filtration and/or a centrifugal separator {(refer to Japanese Patent Application Laid-Open No. 77637/1982 (Show 57), Japanese Patent Application Laid-Open No. 331088/1993 (Hei 5), Japanese Patent Application Laid-Open No. 275539/1988 (Show 63), Japanese Patent Application Laid-Open No. 107578/1994 (Hei 6), Japanese Patent Application Laid-Open No. 306002/1994 (Hei 6) and the like}.
In the above-mentioned solid-liquid separation method, there are usable a suction type belt filter and a drum filter in the separation by means of filtration method {(refer to Japanese Patent Application Laid-Open No. 306002/1994 (Hei 6)}. In this case, even if a high degree of vacuum is applied to suction for the purpose of removing the mother liquor on the surface of the crystals and decreasing intercrystalline liquid content in a short period of time, it is impossible to contrive marked decrease in the liquid content. Further on the other hand, there are brought about filter clogging and/or damage to a filter medium due to the adduct of bisphenol A with phenol depending upon the degree of opening of the selected filter or the material of the filter medium, thereby causing an obstacle to normal operation. On the contrary, an extremely low degree of vacuum, when applied to suction, requires a markedly long time in separating the mother liquor from the adduct of bisphenol A with phenol. In addition, impurities in the mother liquor are entrained into the next separation step and at the same time, are responsible for an increase in operational load and deterioration in the product quality. Moreover since the slurry of the reaction mixture and cleaning liquid are composed principally of phenol, unfavorable phenomena take place such that phenol is precipitated at a temperature lower than 30° C. and the adduct of bisphenol A with phenol adheres to a filter medium.
The method using a centrifugal separator {Japanese Patent Application Laid-Open NO. 107578/1994 (Hei 6), Japanese Patent Application Laid-Open NO. 306002/1994 (Hei 6)} is preferable with respect to decreasing intercrystalline liquid content and affording more dried product, but the substitution efficiency of the mother liquor, cleaning liquid or the like is made inferior to the substitution efficiency of the filtration method by the occurrence of crystal disintegration and the like due to centrifugal load imposed thereon. For these reasons, in order to improve the purity of the product in the case of treating a large amount of product, it is indispensable to generally repeat cleaning thereof by the use of a plurality of machines and instruments, whereby economically unfavorable results are brought about by increase in the number of machines and instruments and prolonged running hours.